1. Field of the Invention
The present invention relates to a hydraulic circuit of winches for a crane for driving two winches equipped in a mobile crane.
2. Description of Related Art
In general, two winches of a main winch and a supplementary winch are installed in a mobile crane. Hydraulic circuits for driving these two winches with using two hydraulic motors and two hydraulic pumps include the two-pump merging series circuit type and the one-pump and one-motor independent circuit type. In the one-pump and one-motor independent circuit type, pressure oil discharged from one hydraulic pump is independently supplied to one hydraulic motor. In the two-pump merging series circuit type, pressure oils discharged from two hydraulic pumps are merged by a series circuit and supplied to hydraulic motors. The two-pump merging series circuit type among these two circuit types is often adopted for a conventional mobile crane since the two-pump merging series circuit type can ensure high outputs. However, due to the characteristic of the series circuit in which a return oil of one hydraulic motor is fed to the other hydraulic motor, there is a disadvantage that the two hydraulic motors influence over each other.
Thus, the present inventors proposed to selectively use the one-pump and one-motor independent circuit type and the two-pump merging series circuit type in accordance with work contents or the like. This proposal is described in Japanese Patent Laid-Open No. 2007-254055. Hereinafter, a part of this proposal shown in FIGS. 8 and 9 will be described.
In this prior art, the configuration of a drive system of a hydraulic circuit is the same as that of the two-pump merging series circuit type. As shown in FIGS. 8 and 9, the hydraulic circuit of this prior art is provided with a first hydraulic motor 1 for driving a first winch such as a main winch, a second hydraulic motor 2 for driving a second winch such as a supplementary winch, first and second hydraulic pumps 3, 4, and first and second control valves 11, 12 for merging pressure oil discharged from both the hydraulic pumps 3, 4 and supplying the pressure oil to the first and second hydraulic motors 1, 2 in accordance with operation of operation levers 21, 23. In the first control valve 11, a first-gear spool 13 for a first winch and a second-gear spool 14 for a second winch are connected in series. The first-gear spool 13 for the first winch is stroked by secondary pressure (pilot pressure) of a first remote control valve 22 operated by the first operation lever 21. The second-gear spool 14 for the second winch is stroked by secondary pressure of a second remote control valve 24 operated by the second operation lever 23. In the second control valve 12, a second-gear spool 15 for the first winch and a first-gear spool 16 for the second winch are connected in series. The second-gear spool 15 for the first winch is stroked by the secondary pressure of the first remote control valve 22 operated by the first operation lever 21. The first-gear spool 16 for the second winch is stroked by the secondary pressure of the second remote control valve 24 operated by the second operation lever 23.
Meanwhile, a control system of the hydraulic circuit includes the first remote control valve 22 and the second remote control valve 24 as shown in FIGS. 8 and 9. The first remote control valve 22 generates the secondary pressure in accordance with the operation of the first operation lever 21 in the wind-up direction or the wind-down direction. The second remote control valve 24 generates the secondary pressure in accordance with the operation of the second operation lever 23 in the wind-up direction or the wind-down direction. Pilot pipe lines 31, 32, 33, 34 provide communication between pilot operation portions 14a, 14b, 15a, 15b of valve sections of the second-gear spools 14, 15 of the control valves 11, 12 and the remote control valves 22, 24. Four second-gear blocking switching valves 41, 42, 43, 44 are respectively provided in second-gear spool side diverged pipe lines 31b, 32b, 33b, 34b of the pilot pipe lines 31, 32, 33, 34. The second-gear blocking switching valves 41 to 44 are collectively switched by a selection switching valve 45 with using primary pressure of the remote control valves 22, 24 (that is, pressure of a pilot hydraulic source 26). The second-gear blocking switching valves 41 to 44 are switchable between transmission positions and block positions. The transmission positions are for transmitting the secondary pressure of the corresponding remote control valves 22, 24 to the pilot operation portions 14a, 14b, 15a, 15b of the valve sections of the second-gear spools 14, 15 of the control valves 11, 12. The block positions are for blocking the transmission of the secondary pressure and for providing communication between the pilot operation portions 14a, 14b, 15a, 15b and a tank 27. The selection switching valve 45 is an electromagnetic control valve for switching the second-gear blocking switching valves 41 to 44 to the transmission positions at an ‘A’ position to be electromagnetically turned OFF, and switching the second-gear blocking switching valves 41 to 44 to the block positions at a ‘B’ position to be electromagnetically turned ON. In the case where the second-gear blocking switching valves 41 to 44 are switched to the transmission positions by this selection switching valve 45, the remote control valves 22, 24 generate the secondary pressure in accordance with the operation of the operation levers 21, 23. The secondary pressure is transmitted to pilot operation portions 13a, 13b, 16a, 16b of valve sections of the first-gear spools 13, 16 of the control valves 11, 12. Further, the secondary pressure is also transmitted to the pilot operation portions 14a, 14b, 15a, 15b of the valve sections of the second-gear spools 14, 15 of the control valves 11, 12. Therefore, the first-gear spools 13, 16 and the second-gear spools 14, 15 are stroked in the operating direction of the operation levers 21, 23 in accordance with operation amounts of the operation levers 21, 23, respectively. Thereby, the two-pump merging series circuit type is realized. Meanwhile, in the case where the second-gear blocking switching valves 41 to 44 are switched to the block positions by the selection switching valve 45, the remote control valves 22, 24 generate the secondary pressure in accordance with the operation of the operation levers 21, 23. The secondary pressure is transmitted to the pilot operation portions 13a, 13b, 16a, 16b of the valve sections of the first-gear spools 13, 16 of the control valves 11, 12 but not transmitted to the pilot operation portions 14a, 14b, 15a, 15b of the valve sections of the second-gear spools 14, 15 of the control valves 11, 12. Therefore, the second-gear spools 14, 15 of the control valves 11, 12 are not stroked irrespective of the operation of the operation levers 21, 23. Thereby, the one-pump and one-motor independent circuit type is realized.
In the case of the two-pump merging series circuit type, as the relationship between the operation amounts of the operation levers 21, 23 (also called as the operation amounts of the remote control valves) and spool strokes, the first-gear spools 13, 16 firstly start stroking and reach the full stroke (the first gear), and then the second-gear spools 14, 15 start stroking and reach the full stroke (the second gear). Meanwhile, in the case of the one-pump and one-motor independent circuit type, only the first-gear spools stroke. Therefore, in the case where this one-pump and one-motor independent circuit type and two-pump merging series circuit type are selectively used, and when the one-pump and one-motor independent circuit type is selected, an invalid stroking area where the spool stroke is not changed in accordance with the operation of the operation levers 21, 23 is increased, so that the operation performance of the operation levers 21, 23 is deteriorated. The prior art is to decrease the invalid stroking area and have the same secondary pressure of the remote control valves (that is, the same operation amounts of the remote control valves) at a stroke start point of the first-gear spools in both the circuit types so as to improve the operation performance of the operation levers 21, 23. In addition to the configuration described above, the hydraulic circuit of the prior art is provided with four constant pressure reduction valves 51, 52, 53, 54, four secondary pressure switching valves 56, 57, 58, 59, a pressure reduction valve 70, four spool stroking start point shifting switching valves 71, 72, 73, 74, and four switching control switching valves 76, 77, 78, 79. The four constant pressure reduction valves 51, 52, 53, 54 reduce the secondary pressure of the remote control valves 22, 24 at constant rates respectively. The secondary pressure switching valves 56, 57, 58, 59 respectively transmit the secondary pressure of the remote control valves 22, 24 to the pilot operation portions 13a, 13b, 16a, 16b of the valve sections of the first-gear spools 13, 16 of the control valves 11, 12 in the case where the second-gear blocking switching valves 41 to 44 are switched to the transmission positions by the selection switching valve 45 (that is, in the case where the two-pump merging series circuit type is selected). The second pressure switching valves respectively transmit secondary pressure of the constant pressure reduction valves 51 to 54 to the pilot operation portions 13a, 13b, 16a, 16b of the valve sections of the first-gear spools 13, 16 of the control valves 11, 12 in the case where the second-gear blocking switching valves 41 to 44 are switched to the block positions by the selection switching valve 45 (that is, in the case where the one-pump and one-motor independent circuit type is selected). The pressure reduction valve 70 reduces the hydraulic pressure of the pilot hydraulic source 26 as primary pressure to predetermined pressure. The spool stroking start point shifting switching valves 71, 72, 73, 74 are respectively provided on the downstream sides of the secondary pressure switching valves 56 to 59 of first-gear spool side diverged pipe lines 31a to 34a of the pilot pipe lines 31 to 34. The switching control switching valves 76, 77, 78, 79 control switching actions of the shifting switching valves 71 to 74.
However, in the prior art above, there is a need for providing the four second-gear blocking switching valves 41 to 44, the four constant pressure reduction valves 51 to 54, the four secondary pressure switching valves 56 to 59, the four spool stroking start point shifting switching valves 71 to 74, and the four switching control switching valves 76 to 79 in order to control the secondary pressure of the remote control valves 22, 24. Therefore, since the hydraulic circuit is complicated, there are various problems in terms of implementation such as high cost.